Magnetic abrasive wheels, method of making them, and machine tools for using them



June 11, 1963 COES, JR 3,093,464

MAGNETIC ABRASIVE WHEELS, METHOD OF MAKING THEM, AND MACHINE TOOLS FORusmc THEM Filed Jan. 16, 1959 AMI.

if M 8 INVENTOR /6 L OE/NG C055 JE.

United States Patent M 3,693,464 MAGNETIC ABRASIVE WHEELS, METHUD 0FMAKING THEM, AND MACHHJE TGOLS FOR USING TIEM Luring Coes, Jr.,Brookfield, Mass, assignor to Norton Company, Worcester, Mass, acorporation of Massachusetts Filed Jan. 16, 1959, Ser. No. 787,216 4Claims. (Cl. 51-295) The invention relates to magnetic abrasive wheels,method of making them and machine tools for using them.

One object of the invention is to provide a grinding wheel or lappingwheel of good vitrified abrasive characteristics which :has a goodmagnetic permeability, i.e. will affect a magnetic field. Another objectof the invention is to provide a grinding or lapping wheel of highmagnetic susceptibility which also has good vitrified abrasivecharacteristics. Another object of the invention is to provide a simpleprocess for the manufacture of such wheels and other abrasive products.Another object is to provide an abrasive wheel for use in precisiongrinders and lapping machines of magnetic properties to act as acomponent in controlling the operation of the machine. Another object ofthe invention is to provide dense wheels and wheels of fine abrasivesize with the characteristics indicated. Another object is to provideorganic bonded grinding wheels and other abrasive products which aremagnetic. Any organic bond can be used such as phenolic resin, shellac,natural and synthetic rubber. Another object is to provide magneticgrinding wheels or other abrasive products bonded with inorganic bondswhich mature at a low temperature. Examples are magnesium oxyc-hlorideand silicate bonds.

Another object is to provide machine tools and electronic componentswith a magnetically susceptible abrasive wheel for controlling grindingoperations. Another object is to provide a combination ofinstrumentalities including a magnetically susceptible abrasive productfor controlling an abrasive operation with precision.

Other objects will be in part obvious or in part pointed outhereinafter.

In the accompanying drawings,

FIGURE 1 is a magnified sectional view of an abrasive wheel according tothe invention,

FIGURE 2 is a block and wiring diagram of a thread grinding machine toillustrate one application of the abrasive wheel of the invention,

FIGURE 3 is a cross sectional view of a furnace which can be used toproduce the abrasive wheel of the invention.

Although abrasive grains are bonded with many different kinds of bond toproduce grinding wheels and lapping wheels, for use in machine tools forprecision grinding and lapping, the vitrified bond has for many yearsbeen preferred. Abrasive grains are bonded with vitrified bond by mixingthem with any of a combination of clays, fluxes such as feldspar, frits(crushed glass of known composition according to formula for theproduction of bond of specific composition). Vitrified bond is acombination of oxides, with usually a large proportion of silica so thatmost vitrified bonds are silicates. They are analyzed as a combinationof oxides of which, besides silica SiO alumina A1 0 is usually prominentalong with MgO, CaO, Na O, K 0, B 0 and frequently some TiO Iron oxideis usually kept at a low figure because of its excessive fluxingproperties and because it discolors the wheels irregularly. It alsogives a bond somewhat unpredictable characteristics in firing.

The invention provides a thoroughly practical vitrified bonded abrasivewheel, a grinding wheel or a lapping wheel or according to any otherdesignation, of free cutting precision quality which has at the sametime 3,093,464 Patented June 11, 1963 2 magnetic properties, that is itis permeable and will affect a magnetic field. It is easy to manufactureas the following description will show. The magnetic material does notdeleteriously atfect the grinding, lapping or other abrasive properties.

According to the invention the magnetic material may be magnetite, Fe OThis is sometimes said to be a combination of one mol of FeO plus onemol of Fe O but is usually written Fe O This oxide is magnetic.

I reject iron because it too readily oxidizes to a nonmagnetic oxide andfurthermore its presence in the .pores of a grinding wheel would causesmearing of the work piece being ground which would be undesirable. Themagnetic metals cobalt and nickel, singly or in combination, can howeverbe used as they do not have bad smearing properties and furthermore dontreadily oxidize to non-magnetic oxides.

Instead of magnetite Fe O I can use cobaltous cobaltic cobalt oxide, C00 or cobalt ferrite, CoO-Fe O or nickel ferrite NiO-Fe O Nickel doesnthave any magnetic oxide of its own. Mixtures of the twometals with thethree oxides in any combinations can be both produced and used.

To produce a grinding wheel according to the invention or to produce anyother type of abrasive wheel according to the invention, I first procurea vitrified grinding wheel manufactured in accordance with any knowntechnique. Nearly all vitrefied abrasive wheels are porous to someextent, enough for the present purposes.

As the greater part of precision grinding and lapping is done withvitrefied wheels having fused alumina abrasive which means that theabrasive is alumina A1 0 of the alpha variety which has been fused in anelectric furnace, I will first describe the invention in connection withsuch a wheel.

EXAMPLE I I procured a grinding wheel which was made of white aluminumoxide abrasive grains of a purity of better than 98%, half of it being280 grit size and the other half (by weight) being 320 grit size, andhaving a volume percentage of abrasive of 47.3%, a volume percentage ofbond of 15.0%, and a volume percentage of pores of 37.7%. These poreswere interconnected. The bond formula is given in the following table.

This wheel was 12" in diameter, /1" thick and had a 3" central hole. Itwas made in the usual way, by coating the abrasive grains with a watersolution of dextrine and then mixing therewith the bond in dry powderedform, molding, firing.

I placed this wheel in a controlled atmosphere furnace heated to 400 C.A stream of nitrogen gas saturated with iron carbonyl vapor was passedthrough the furnace at the rate of two litres per minute for two hours.Then the furnace was flushed out with nitrogen and steam was passedthrough it at the rate of two litres per minute for two more hours, thetemperature still being held at 400 C. The furnace was then allowed tocool and when the aceaaee wheel was cool it was complete. Naturally morethan one wheel will be made at one time.

FIGURE 1 shows the condition of this wheel. The fused aluminum oxideabrasive grains of 280 and 320 grit size are indicated by the numeral 1.The bond is indicated by the numeral 2, the pores by the numeral 3. Thecoating of mangetite Pe o, on the pore walls is indicated by the numeral4 and the wheel as a whole is indicated by the numeral 5. This wheel issufliciently magnetically permeable for the purposes of this invention.

The furnace used in making the wheel of Example I is shown in FIGURE 3.It consisted of a chamber comprising a bottom 6 and a top 7 connected bya sand seal 8 with an inlet pipe 9 an outlet pipe 10 and a bar 11 tohold the wheels 5. The inside diameter of the bottom 6 was 24" and theoverall height of the furnace was 44" and the other dimensions wereabout in proportion thereto as shown in FIGURE 3. All parts except thesand were made of stainless steel. The furnace was cylindrical.

Wheels according to the invention are useful for controlling grindingoperations as illustrated in the diagram of FIGURE 2. In FIGURE 2 theWork piece 12 is being ground by a grinding wheel 5 made in accordancewith the invention. In Example I the manufacture of a straight wheel wasdescribed, but this can be trued to a shape like that shown in FIGURE 2and this is the way to make such a shape. The wheel 5 is mounted on aspindle 13 which is rotated as by means of belts 14. As FIGURE 2 issimply a diagram, a feed nut 15 is represented which is connected to aslide 16 to move the spindle 13 forward and back in the direction of thework, the spindle 13 being journalled on the slide 16. The nut 15 ismoved by a screw shaft 17 driven by an electric motor 18. Referring nowto the lower right of FIGURE 2 and working to the left and up, thearmature of the motor 18 is energized by power lines 20 through a doublerelay switch 21, another double relay switch 22, lines 23, a slidecontrolled switch 24 and lines 25. The slide controlled switch 24 has aninsulated element attached to the slide itself which, when it withdrawsto a certain point, opens the circuit as diagrammatically indicated. Inthis description the abrasive product is moved relative to the workpiece as a result of the action of the magnetic sensing device, butsince it is only the relative motion between the abrasive and the workthat is necessary, it is obviously possible operatively to position thesensing device to move the work piece relative to the abrasive, or tomove both, in order to achieve the desired result.

Referring now to the upper right hand part of FIG- URE 2 and working tothe left, the stopping and starting of the motor 18 is controlled bypower lines 28 to an ofl? and on control 29 energized by an amplifier 30which receives its signal from a bridge 31 energized by an oscillator32. The bridge 31 is connected by wires 34 and 35 to a magnetic sensingdevice 36. This magnetic sensing device carries more current with agiven E.M.F. when the wheel 5 comes nearer to it on account of themagnetic permeability of the wheel 5. When the device 36 carries morecurrent, the bridge 31 has a greater output. The input to the bridge 31is from the oscillator 32. The circuit of the bridge 31 is from the wire34 to the device 36 to the wire 35 through a variable resistance 40through a resistance 41 and a variable condenser 42 through a variableinductance 43 and back to the wire 34. As controlled by the setting ofthe bridge 31, when the wheel 5 has moved a certain distance into thework piece 12, the output from the bridge 31 is amplified by theamplifier 30 to a high enough value to cause the off and on control 29to actuate the relay solenoid 44 which opens the switch 22 and stops theinfeed. But when the wheel 5 has been worn away some thus becoming ofless diameter, the Spindle 13 has to advance farther to actuate the ofiand on control 29. The face of the wheel 5 will be at the same spot. Inthread grinding machines, the grinding wheels are trued at frequentintervals, but this mechanism in combination with a magneticallysusceptible wheel lbrings the wheel every time to a position to cut thethread in the work piece 12 to the same depth. The magnetic sensingdevice 36 is one or more coils or the equivalent.

The arrangement shown in FIGURE 2 is particularly useful for threadgrinding of all kinds and can also be used for surface grinding andcylindrical traverse grinding. Since this invention is in a grindingwheel structure and a method of producing it I dont need completely todescribe the machine, since the above description is merely to show theutility of the wheel and such machines actually exist.

However the machine should have a circuit to cause the slide 24 towithdraw at the end of a grinding operation. Referring to the lowerright of FIGURE 2, a push button switch 45 is connected by lines 46 tothe lower lines 20 and when closed, through lines 47 energizes a relaysolenoid 48 which closes a double relay switch 49 and opens the doublerelay switch 21. The double relay switch 49 is connected by lines 50 tothe power lines 20 and by lines 51 to the lines 23, and it will be seenthat the direction of the current is reversed through the double relayswitch 49 as compared with through the double relay switch 21. The motor18 is a reversible motor. When the push button switch 45 is closed thearmature circuit is reversed so that the slide 16 retreats, and when itgets to a certain position the switch 24 opens the circuit and the motor18 stops thus stopping the withdrawal of the slide. Referring to thebottom middle left of FIG- URE 2, to start the machine up again theoperator momentarily presses a push button switch 55 which connectslines 56 to lines 57, the former being connected to lines 23 and thelatter to lines 25. Later the circuit is reestablished through the slideswitch 24. Similarly, my combination of a magnetic abrasive product anda magnetic sensing device operating relatively to feed the abrasivemember and work piece toward and away from each other may be readilyapplied to abrasive operations involving segments, discs, sticks andother shapes in surface grinding and other types of abrading operationsas well as cylindrical grinding.

EXAMPLE II 1 coated pores of the same kind of a grinding wheel asspecified in Example I with magnetite, Fe O in the following manner:

The wheel, the same size and composition and volume percentage ofabrasive bond and pores 'as in the case of Example I, was placed in atightly fitting can, open at the top, and was covered with a 30% watersolution of ferric nitrate, Fe(NO although other water soluble salts ofiron could be used. This can was then placed in a closed containerhaving an atmosphere of ammonia and the can with the wheel in itimmersed in the ferric nitrate solution was kept in this atmosphere ofammonia for three days. The ammonia NH reacted with the water to produceammonium hydroxide NH OH, which reacted with the ferric nitrate toproduce ammonium nitrate NH NO and iron hydroxide Fe(OH) This caused theprecipitation of the iron hydroxide in the pores of the wheel leaving awater solution of ammonium nitrate also in the pores with very littleelse. This wheel was then dried at C. for twenty-four hours whichprecipitated the ammonium nitrate into the pores.

The wheel was then fired in a muffle furnace at 550 C. for four hoursduring which time an atmosphere of steam was maintained in the furnace.The steam removed the ammonium nitrate and converted the ferrichydroxide to magnetite Fe O This wheel weighed about 6% more than it didoriginally and had a magnetic susceptibility of 40x110- c.g.s. units.This wheel is also fully illustrated in FIGURE 1 and is usable asalready described for the wheel of FIGURE 1. The furnace of FIGURE 3 isa mufile furnace which is used to fire this wheel of Example II. Forpurposes of complying with the patent statute, this example is selectedas representing the best mode for vitrified bonded wheels. I

EXAMPLE III Vitrified grinding wheels have their pores coated withcobalt ferrite Coo-R by proceeding as in Example II substituting asolution containing cobaltic nitrate and ferric nitrate for the 30%ferric nitrate of Example II. All other steps are identical.

EXAMPLE IV In order to coat the pores of la vitrified wheel with nickelferrite, NiO-Fe o I proceed as in Example II substituting a watersolution containing 10% of nickel nitrate and 20% :of ferric nitrate.

EXAMPLE V In order to coat the pores of [a vitrified wheel withcobaltous cobaltic oxide, C0 0 I proceed as in Example 11 but use a 30%water solution of cobaltic nitrate, am- 3h- All of these wheels aremagnetic, but I have only measured the magnetic susceptibility of thewheel made according to Example II. This had a magnetic susceptibilityof 40x10" c.g.s. units. For use in accordance with this invention amagnetic susceptibility of at least 10- c.g.s. units may be satisfactoryfor operating some equipment of this type. While there is no upperlimit, it would be extremely diificult to coat the pores of a grindingwheel to give it a magnetic susceptibility of more than 600x 10- c.g.s.units. The examples herein produce wheels having a susceptibility ofmore than l0 l0- c.g.s. units which is desirable.

EXAMPLE VI For the manufacture of a grinding wheel having in the poresthereof cobalt metal, I may proceed in accordance with Example I usingcobalt carbonyl vapor instead of iron carbonyl vapor, stopping theprocess after passing the cobalt carbonyl vapor through the furnace inthe stream of nitrogen gas. This causes the deposit of cobalt in thepores of the wheel.

EXAMPLE VII For the manufacture of a grinding wheel having in the poresthereof nickel metal, I may proceed in accordance with Example I usingnickel carbonyl vapor instead of iron carbonyl vapor, stopping theprocess after passing the nickel carbonyl vapor through the furnace inthe stream of nitrogen gas. This causes the deposit of nickel in thepores of the wheel.

EXAMPLES VIII AND IX In order to coat the pores of a wheel with cobalt,I may proceed 'as in Example II using a 30% water solution of cobaltnitrate (instead of ferric nitrate) firing the wheel in the mufiiefurnace at 550 C. for four hours in an atmosphere of nitrogen withoutany steam. The cobalt hydroxide is converted to cobalt metal and theammonium nitrate is driven off. In the same way the pores of a wheel canbe coated with nickel, substituting nickel nitrate for the ferricnitrate of Example LI and proceeding as in Example II through the firingin the muffle furnace in an atmosphere of nitrogen without any steam.

FURTHER EXAMPLES Mixtures and complexes of the oxides and the metals canbe deposited on the surfaces of the pores of a grinding wheel byproceeding in accordance with the principles of Example II and using asingle nitrate mentioned herein or mixtures of these nitrates and firingin the muffle furnace for a short time in steam, the rest of the time inan atmosphere of nitrogen. Mixtures of metals can be deposited usingmixtures of nitrates and [firing in nitrogen without steam.

I have given 400 C. as the temperature for heating the abrasive wheel incarbonyl vapor. This temperature must be above about 150 C., which isthe approximate decomposition point of the carbonyl vapor to form metal.The upper limit for use in my process is indefinite because otherfactors come into consideration such as the initial temperature at whichthe wheel was fired, the ease of apparatus operation, etc. ordinarily toavoid premature decomposition of the carbonyl, the temperature shouldnot be much above 500 C. However the grinding wheel should not bedeformed by the heat. The non-oxidizing atmosphere mentioned throughoutis nitrogen gas. However any other non-oxidizing atmosphere can be usedincluding the inert gases, argon, helium, etc. The oxidizing atmospherespecified herein is steam. However other oxidizing atmospheres such asoxygen itself can be used. Ordinary air can be used but more time wouldbe needed to effect the conversion.

For coating the pores of vitrified wheels using nitrate solutions, Ihave specified 550 C. for firing the wheels in a muffie furnace toconvert the hydroxide to magnetic oxide or metal or combination.Ordinarily, a temperature above 400 C. would be used to decompose thehydroxide and temperatures above about 1000 C. are undesirable becauseof possible loss of magnetic properties through the formation of ferrousoxides.

It has recently been found that for precision grinding if high wheelspeeds are used as measured in surface feet per minute (s.f.p.m.),organic bonded grinding wheels give superior performance in many cases,such as in the thread grinding described. Organic bonded grindingWheels, especially phenolic resin bonded grinding wheels, can beoperated at higher speeds measured in s.f.p.m. than vitrified bondedwheels because the former are stronger. This more than overcomes thesuperior cutting rate of vitrified bonded wheels at a given s.f.p.m.This has caused the development of new grinders capable of operating athigher wheel speeds, but these are now available in some quantities.

So far as the machine tool combination of the present invention isconcerned, or other abrasive member, the grinding wheel can be made withvarious bonds. Of the organic bonds, phenolic resin is usually preferredfor grinding operations of the type herein disclosed. This wheel is madeby incorporating into the wheel mixture any one or combination of themagnetically susceptible materials previously listed in sufiicnientquantities to give the wheel a magnetic susceptibility of at least 10-c.g.s. units.

EXAMPLE X A wheel 8" in diameter, /2" thick with a /2" central hole wasmade in the following manner. The ingredients were:

Weight/VOL, 2.92.

The fused alumina of mesh grit size), was first wet with 20 cc. ofresinper pound of bond, then phenolic resin which was in powder form andof the brand BR2417 together with the magnetite and the calcium oxide(to take up water during curing) were added and thoroughly mixedwhereupon 10 cc. of 'anthracene oil per pound of bond was added tosettle the dust. This mixture was cold molded in a closed mold and bakedfor twenty-four hours at C. The wheel had a magnetic susceptibility ofabout 60x10 c.g.s.

For purposes of complying with the patent statute, this Example X isselected as the best mode for organic bonded wheels.

Other magnetic material besides those already mentioned can be used inthis phase of the invention. Among these are various magnetic ferrites,iron and its alloys, aluminum-nickel-cobalt alloys, copper-manganesealloys and rare earth garnets can be used. They are preferably providedin finely powdered form to smooth out mixing procedures and avoiddifiiculties of poor distribution. The reason why many othermagnetically susceptible materials can be used is that the temperatureof baking is very low in comparison with the temperature of vitrifying.Since wheels are usable in the invention if they have the mum magneticsusceptibility of at least 10 c.g.s. units, it is unnecessary to definea complete group of the magnetic materials which do not come under anyrecognized classification with respect to reactivity properties.

Other commonly used organic bonds can be employed to make grindingwheels or other abrasive products as components of the apparatus of theinvention. Such bonds include shellac and natural or synthetic rubber.Also such inorganic bonds for grinding wheels as magnesium oxychlorideand silicate, by which is meant bonds based on the use of alkalisilicate as a primary bonding ingredient in the mixture, may beemployed, and the wheels are made magnetic by the same procedure usedfor organic wheels. These wheels are made by methods known to the artwith the addition of the magnetically susceptible material as a filler.Metal bonded wheels are readily made magnetic by using magnetic metalsas a constituent of the bond.

It will thus be seen that there has been provided by this inventionmagnetic abrasive wheels and other abrasive products, methods of makingthem and machine tools for using them in which the various objectshereinabove set forth together with many thoroughly practical advantagesare successfully achieved. As many possible embodiments may be made ofthe above invention and as many changes might be made in the embodimentsabove set forth, it is to be understood that all matter hereinbefore setforth, or shown in the accompanying drawings, is to be interpreted asillustrative and not in a limiting sense.

I claim:

1. A porous vitrified grinding wheel having on the surfaces of the poresthereof magnetically susceptible material seleoted from the groupconsisting of magnetite Fe O cobaltous cobaltic oxide C0 0 cobaltferrite CoO-Fe O nickel ferrite NiO'Fe O cobalt and nickel and mixturesthereof, and the wheel having a magnetic susceptibility of at least 10-c.g.s. units.

2. A porous vitrified grinding wheel according to claim 1 in which thematerial is magnetite.

3. A porous vitrified grinding wheel according to claim 1 in which thematerial is cobalt.

4. A porous vitrified grinding wheel according to claim 1 in which thematerial is nickel.

References Cited in the file of this patent UNITED STATES PATENTS1,187,431 King June 13, 1916 2,038,727 Geyer Apr. 28, 1936 2,052,194Sandorff Aug. 25, 1936 2,114,160 Whitacre Apr. 12, 1938 2,114,166 LeeuwApr. 12, 1938 2,125,782 Heald Aug. 2, 1938 2,164,476 Scutt July 4, 19392,377,995 Coes June 12, 1945 2,426,139 Bishop et al Aug. 19, 19472,657,505 Price Nov. 3, 1953

1. A POROUS VITRIFIED GRINDING WHEEL HAVING ON THE SURFACES OF THE PORESTHEREOF MAGNETICALLY SUSCEPTIBLE MATERIAL SELECTED FROM THE GROUPCONSISTING OF MAGNETITE FE3O4, COBALTOUS COBALTIC OXIDE CO3O4, COBALTFERRITE COO.FE2O3, NICKEL FERRITE NIO.FE2O3, COBALT AND NICKEL ANDMIXTURES THEREOF, AND THE WHEEL HAVING A MAGNETIC SUSCEPTIBILITY OF ATLEAST 10**6 C.G.S. UNITS.